Topographic measurement using stereoscopic picture frames

ABSTRACT

Disclosed is a topographic measurement system wherein at least one satellite is used to scan the earth surface and send picture frames of a target area captured at different positions to an earth station. The picture frames are combined to produce a number of pairs of frames which constitute a stereoscopic image of the target area. Each frame pair is analyzed according to a number of visual characteristics and evaluated with a set of fitness values representative of the degrees of fitness of the frame pair to topographic measurement of the target area. A total of the fitness values is obtained from each frame pair and compared with the total values of other frame pairs. A frame pair having the highest total value is selected as a best pair. A parallax between the best pair frames is determined to produce first and second sets of line-of-sight vectors for conversion to topographic data.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 10/600,087,filed Jun. 20,2003 in the name of Makoto MARUYA and entitled TOPOGRAPHICMEASUREMENT USING STEREOSCOPIC PICTURE FRAMES.

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No.: 2002-180064 filed Jun. 20,2002, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to topographic measurement of atarget area using image sensors mounted on flying vehicles such as earthobservation satellites.

2. Description of the Related Art

Topographic measurement using stereoscopic pictures is known as remotesensing technology. In one topographic measurement system known as across-track stereoscopic imaging system, a single image sensor (HRV andAVNIR sensors) mounted on a satellite (SPOT and ADEOS satellites) isused to capture a number of pictures of a target area at different timeswhen the satellite is encircling on separate orbits. In other systemknown as an along-track imaging system, use is made of two image sensors(OPS and PRISM sensors) on board a single satellite (JERS-1 and ALOSsatellites) to capture multiple pictures of a target area at differentangles when the satellite is encircling on the same orbit over thetarget area. While the latter is able to send pictures at frequentintervals, hence available stereoscopic pictures can be easily obtained,a high-capacity memory is required on board the ship to store picturesbefore transmission to the earth. However, in applications where a highresolution of one meter is desired, the cross-track stereoscopic imaginghas been preferred to the along-track stereoscopic imaging.

A recent advance in the remote sensing technology is the development ofan earth observation satellite such as IKONOS and Quick Bird satellitesin which a single sensor performs the functions of both cross-trackimaging and along-track imaging systems.

When a pair of stereoscopic pictures is sensed, the pictures are scannedline by line and transmitted from the satellite in the form of frames tothe earth station. The transmitted frames are analyzed in terms ofpoint-to-point correlations between the frames to determine how muchthey differ from one another. This correlation information is known asparallax. Using a model of the image sensor, a position is determined ina three-dimensional coordinate system for each point-to-pointcorrelation. A set of such 3-D position data obtained from a target areaconstitutes topographic data of the target area.

However, in order to sense a target area from an earth observationsatellite, it is necessary to ensure that, when the satellite isapproaching the target area, it is bright under sun light and notshadowed by any cloud. Chances for taking appropriate pictures aretherefore limited. In particular, in applications where high resolutionis desired, a single-sensor, cross-track earth observation satellitewill be used. When the satellite is approaching a target area, thesensor must be pointed toward the target area from different angles atdifferent times to obtain a pair of stereoscopic frames. Therefore, thetarget area must be clear and bright for both chances of image sensing.Additionally, the target area must be pointed from relatively largeangles. This requires that the satellite orbits be distancedsufficiently from each other. During the time the satellite isencircling on intermediate orbits, no appropriate pictures cannot betaken, which leads to a low efficiency of satellite utilization.Therefore, target areas suitable for acquiring stereoscopic pictures aresignificantly limited.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide atopographic data processor and a topographic measurement system whichcan acquire stereoscopic picture frames with high efficiency ofsatellite utilization.

The stated object is obtained by the provision of a frame pair selectorfor selecting a pair of picture frames that constitute a stereoscopicimage from multiple frames which may be stored in a storage medium orreceived from one or more satellites.

According to a first aspect of the present invention, there is provideda topographic data processor comprising means for selecting a pair offrames from a plurality of candidate frames of a target area capturedfrom different high-altitude positions, the pair of frames constitutinga stereoscopic image of the target area, means for determining aparallax between the selected frames and producing therefrom a firstplurality of line-of-sight vectors and a second plurality ofline-of-sight vectors, and means for converting the first and secondpluralities of line-of-sight vectors to topographic data.

A best frame pair is selected by first forming candidate picture framesinto a plurality of pairs of stereoscopic frames and then evaluating theframe pairs with fitness values representative of their fitness totopographic measurement and selecting a best frame pair having thehighest fitness value.

According to a second aspect, the present invention provides atopographic data processor comprising frame selecting means forselecting a pair of frames from a plurality of candidate frames of atarget area captured from high-altitude positions, the selected pair offrames constituting a stereoscopic image of the target area. Schedulingmeans is provided for selecting at least one airborne image sensor if anappropriate frame is not available in the plurality of candidate framesand sensing picture frames from the selected image sensor, whereby theframe selecting means uses the sensed frames to select a pair of frames.A parallax calculation means is provided for determining a parallaxbetween the frames selected by the frame selecting means and producingtherefrom a first plurality of line-of-sight vectors and a secondplurality of line-of-sight vectors. The first and second pluralities ofline-of-sight vectors are converted to topographic data.

According to a third aspect of the present invention, there is provideda topographic measurement system comprising at least one image sensormounted on a vehicle flying over a target area, a receiver for receivinga plurality of picture frames captured by the image sensor at differentpositions, means for selecting a pair of frames from the plurality offrames, the pair of frames constituting a stereoscopic image of thetarget area, means for determining a parallax between the selectedframes and producing therefrom a first plurality of line-of-sightvectors and a second plurality of line-of-sight vectors, and means forconverting the first and second pluralities of line-of-sight vectors totopographic data.

According to a fourth aspect of the present invention, there is provideda topographic measurement system comprising at least one image sensormounted on a vehicle flying over a target area, a receiver for receivinga plurality of picture frames captured by the image sensor at differentpositions, frame selecting means for selecting a pair of frames from theplurality of frames of the target area, the selected pair of framesconstituting a stereoscopic image of the target area, scheduling meansfor selecting at least one image sensor if an appropriate frame is notavailable in the plurality of frames, sensing picture frames from theselected image sensor, whereby the frame selecting means uses the sensedframes to select a pair of frames, means for determining a parallaxbetween the frames selected by the frame selecting means and producingtherefrom a first plurality of line-of-sight vectors and a secondplurality of line-of-sight vectors, and means for converting the firstand second pluralities of line-of-sight vectors to topographic data.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in detail further with referenceto the following drawings, in which:

FIG. 1 is a schematic diagram of a land observation system according toa first embodiment of the present invention;

FIG. 2 is a block diagram of a topographic data processor of the firstembodiment of the present invention;

FIG. 3 is a block diagram of the frame pair selector of the topographicdata processor;

FIGS. 4 and 5 are schematic diagrams useful for describing the operationof the geometric condition analyzer of the topographic data processor;

FIG. 6 is a block diagram of the parallax calculator of the topographicdata processor;

FIG. 7 is a schematic diagram useful for describing the operation of theparallax calculator;

FIG. 8 is a flowchart useful for describing the operation of the firstembodiment of the present invention;

FIG. 9 is a block diagram of a topographic data processor of a secondembodiment of the present invention;

FIG. 10 is a block diagram of an image sensing scheduler of FIG. 9;

FIG. 11 is a schematic diagram useful for describing the operation of ageometric condition analyzer of FIG. 10; and

FIG. 12 is a flowchart useful for describing the operation of the secondembodiment of the present invention.

DETAILED DESCRIPTION

Referring now to FIGS. 1 and 2, there is shown a land observation systemaccording to a first embodiment of the present invention. The systemincludes a first land observation satellite 11 encircling the earth onan orbit 16 and a second land observation satellite 12 encircling theearth on an orbit 17 spaced from the orbit 16. Each satellite isconstantly capturing images of the land surface including a target area13. Specifically, the satellite 11 includes a two-dimensional CCD sensor21 and a high-precision telescope, not shown, for focusing the image ofa land surface onto the CCD sensor 21 and an RF transmitter 23.Likewise, the satellite 12 includes a two-dimensional CCD sensor 22 anda high-precision telescope, not shown, for focusing the image of a landsurface onto the CCD sensor 22 and an RF transmitter 24.

The images (picture frames) captured by both satellites are transmittedon a downlink frequency from the transmitters 23 and 24 to an earthstation 14 where the RF signals are amplified and down-converted tobaseband signals by an RF transceiver 25. The signals received from bothsatellites are stored in a storage medium 27 which may be provided in atopographic data processor 15. In this way, the storage medium 27 storesa series of picture frames of land surfaces captured by satellites 11and 12.

As will be described in detail later, a pair of picture frames whichform a stereoscopic image adequate for the determination of the altitudeof a surface feature of a target area is selected by a frame pairselector 28 from the storage medium 27. The selected pair of pictureframes are supplied to a parallax calculator 29 to detect a parallaxbetween the selected frames. Based on the calculated parallax, theparallax calculator 29 produces a first plurality of line-of-sightvectors from one of the selected frames and a second plurality ofline-of-sight vectors from the other frame. A parallax-terrain converter30 is connected to the output of the parallax calculator 29 to producetopographic data based on the first plurality of line-of-sight vectorsand the second plurality of line-of-sight vectors. The topographic datais supplied to output means 31.

As illustrated in detail in FIG. 3, the frame pair selector 28 comprisesa decision module 40, a frame combiner 41 and a plurality of analyzers42 to 46.

Since the picture frames captured by the satellites may contain imageswhich cover outside of the target area 13, the frame combiner 41 firstselects only those picture frames covering the target area and proceedsto combine picture frames selected from those captured by satellite 11with frames selected from those captured by satellite 12 to form aplurality of frame pairs each composing a stereoscopic image of thetarget area. If a frame consists of red, green, blue and near-infraredlight components, these components are not treated individually in sofar as their resolutions are equal to each other. Since panchromaticimages have in most cases twice as high resolution as red, green andblue components, they are treated separately from the color components.All pairs of frames combined by the frame combiner 41 are supplied toall analyzers 42 to 46 as candidate frame pairs.

Analyzers 42 to 46 perform individual analysis on the candidate framepairs according to different visual characteristics and evaluate eachpair of frames with a set of fitness values Q1 through Q6 each beingrepresentative of the fitness of the frame pair to topographicmeasurement of the target area.

For each frame pair, the decision module 40 scales the fitness valueswith respective weight values w_(i) and totals the weighted fitnessvalues w_(i)Q_(i) to produce a quality value Q of the frame pair. Theprocess is repeated on all frame pairs and finally the decision module40 selects one of the frame pairs having the highest quality value as abest frame pair.

Analyzer 42 is a geometric condition analyzer which evaluates thecandidate frame pairs and determines a weight value Q1 according togeometric conditions of the frame of each pair, such as the resolution(i.e., an area covered by a single pixel and measured in terms ofmeters) or the angles of orientation of the satellites to the landsurface. In a simple yet effective method, a pair of high-resolutionframes is evaluated with a high weight value.

If the frames of a pair are of different resolution, the frame of lowerresolution is chosen to evaluate the pair. The evaluation of a pair withlower resolution is preferred to evaluating the pair with an averagevalue of the resolutions of its frames. In this case, the fitness valueQ1 is given as:Q1=1/Resolution   (1)

If high precision is important for the frame pair evaluation, quantumerrors of the frames of each pair are used to evaluate the pair. Inprinciple, this method involves determining a displacement in athree-dimensional space between the frames of a stereoscopic pair on apixel-by-pixel basis and representing it as a quantum error in a systemof three-dimensional axes.

Assume that the image sensors of satellites 11 and 12 are pointingtoward a target point “o” in a three-dimensional coordinate system (x,y, z) with its x and y axes pointing North and East, respectively, onthe earth surface 18 (which is approximated as a flat plane), and its zaxis pointing skyward, as shown in FIG. 4. The image sensor of satellite11 is located in a position s₁ of altitude h₁ from the ground point g₁,azimuth angle a₁ and angle of elevation e₁, while the image sensor ofsatellite 12 is located in a position s₂ of altitude h₂ from the groundpoint g₂, azimuth angle a₂ and angle of elevation e₂. Ground points g₁and g₂ are at distances b₁ and b₂ from the point of origin o,respectively, and mutually spaced at distance b₃. Points o, g₁ and g₂form a triangle with inner angles a₃, a₄ and a₅, and points o, s₁ and s₂form a triangle, called epipolar plane 50. A portion of the epipolarplane 50 in the neighborhood of the point of origin “o” can be enlargedas shown in FIG. 5 to illustrate its details to the size of pixels.

In FIG. 5, adjacent pixels on the image sensor of satellite 11 areindicated as p₁₁ and p₁₂ from which line-of-sight vectors v₁₁ and v₁₂extend toward the point of origin. Likewise, adjacent pixels on theimage sensor of satellite 12 are indicated as p₂₁ and p₂₂ from whichline-of-sight vectors v₂₁ and v₂₂ extend toward the point of origin.Horizontal distance r₁ between line-of-sight vectors v₁₁ and v₁₂represents the resolution of the image sensor of satellite 11 andhorizontal distance r₂ between line-of-sight vectors v₂₁ and v₂₂represents the resolution of the image sensor of satellite 12.

On the epipolar plane 50, the line-of-sight vector v₁₂ forms an anglee_(1′) to the earth surface 18 and the line-of-sight vector v₂₂ forms anangle e_(2′) to the earth surface 18. The angles e_(1′), e_(2′) and theresolutions r₁, r₂ are given as follows:e _(1′)=arctan {h ₁ /b ₁ cos a ₃}  (2a)e _(2′)=arctan {h ₂ /b ₂ cos a ₄}  (2b)r₁=r_(1′) sin e_(1′)  (3a)r₂=r_(2′) sin e_(2′)  (3b)where r_(1′) and r_(2′) are the resolutions of the sensors of satellites11 and 12 when they directly point to ground points g₁ and g₂,respectively.

The quantum error at the point of origin “o” is represented by an area50A defined by line segments c₁-c₂, c₂-c₃, c₃-c₄ and c₄-c₁. Therefore, astereoscopic image whose area 50A is small is evaluated with a highfitness value since the size of the area 50A determines the resolutionof the image. The quantum error is decomposed into a horizontalcomponent E_(h) and a vertical component E_(v) which are given asfollows:E _(h)=(r _(1′) tan e _(1′) +r _(2′) tan e _(2′))/(tan e _(1′)+tan e_(2′))  (4a)E _(v)=(r _(1′) +r _(2′))(tan e _(1′))(tan e _(2′))/(tan e _(1′)+tan e_(2′))  (4b)

It is seen that the fitness value Q1 of a frame pair is inverselyproportional to its quantum error as given by the following relations:

$\begin{matrix}\begin{matrix}{{Q\; 1} = {1\mspace{14mu}\left( {{{if}\mspace{14mu} E_{h}} < {E_{h\_ req}\mspace{14mu}{and}\mspace{14mu} E_{v}} < E_{v\_ req}} \right)}} \\{= {0\mspace{14mu}({otherwise})}}\end{matrix} & (5)\end{matrix}$where E_(h) _(—) req and E_(v) _(—) req are threshold values of E_(h)and E_(v).

Alternatively, the fitness value Q1 can be determined as:Q1=q _(—)1/E _(h)+1/E _(v)  (6)where q_1 represents a parameter of positive value for giving differentweights to the error components E_(h) and E_(v). In most cases, theparameter q_1 is equal to unity. In comparison with other analyzers, thegeometric condition analyzer 42 plays a significant role for selecting abest frame pair from the candidate frame pairs.

Analyzer 43 is a filtering condition analyzer which evaluates the framepairs from the frame combiner 41 in terms of their weight to topographicmeasurement which varies depending on the filtering characteristics ofthe satellite image sensors. Picture frames obtained by a satelliteimage sensor with the full spectrum of visible light, i.e., panchromaticimages, are usually of high S/N quality and fitting to topographicmeasurement. If two frames are obtained by image sensors of likefiltering characteristics, they are also suitable to form a pair fortopographic measurement since they are less affected by differences infiltering characteristics when a parallax is calculated between them.Therefore, the filtering condition analyzer 43 evaluates the frames ofeach candidate pair with a fitness value Q2 which is proportional to theamount of visible spectral components they have obtained as a result ofthe filtering characteristics of the satellite sensors as well as to thelikeness of their wavelength characteristics to each other. The fitnessvalue Q2 is expressed by the following formula:

$\begin{matrix}{{Q\; 2} = \frac{\int_{S}^{\;}{{f(w)}{g(w)}{\mathbb{d}w}}}{S}} & (7)\end{matrix}$where, S indicates the spectrum of visible light, w is the wavelength,and f(w) and g(w) represent the filtering characteristics of the imagesensors of satellites 11 and 12, respectively (i.e., thetransmissibility of filtered visible wavelengths incident on the imagesensors). The functions f(w) and g(w) are of high value if thetransmissibility of wavelengths is high. Equation (7) thus indicatesthat greater the filtering functions overlap each other the fitnessvalue Q2 becomes higher. The panchromatic image is given the highest Q2value.

Analyzer 44 is a sunlight condition analyzer which evaluates the framesof each candidate pair with a fitness value Q3 in terms of their weightto the calculation of parallax which varies depending on the sunlightcondition under which the frames are captured. If sunlight conditionsunder which frames of a pair are captured are substantially equal toeach other, their images will show similar shadow and shading effects toeach other. Since the frames of like sunlight conditions result in anaccurate parallax, they are evaluated with high fitness value Q3.

The following is an evaluation formula for the fitness value Q3:

$\begin{matrix}\begin{matrix}{{Q\; 3} = {1\mspace{14mu}\left( {{{if}\mspace{14mu}{{a_{1} - a_{2}}}} < {d\mspace{14mu}{and}\mspace{14mu}{{e_{1} - e_{2}}}} < d} \right)}} \\{= {0\mspace{14mu}({otherwise})}}\end{matrix} & (8)\end{matrix}$where, d is a threshold angle of several degrees. Equation (8) indicatesthat Q3 is high if azimuth angles a₁ and a₂ and angles of elevation e₁and e₂ are almost equal to each other. If their differences exceed thethreshold angle, the influence of sunlight conditions on frames remainsconstant.

Analyzer 44 is a time-difference analyzer which evaluates the framepairs from the frame combiner 41 in terms of their weight to topographicmeasurement which varies depending on the time difference between theframes of each pair. Since the time difference between frames (two orthree seconds, for example) may result in objects pictured in one frametaking different shapes and positions from those of the other, a highfitness value Q4 is given to a pair of frames whose time difference issmall. However, the time difference may vary significantly depending onthe orbits of satellites and other factors.

Q4 is given as follows:Q4=exp(−T _(d))  (9)where T_(d) is the absolute value of difference between the picturedtimings of the frames and represented by the number of days. The fitnessvalue Q4 may also be given by observing frame pictures by human eyes. Ifthe pictures of any two frames differ substantially, a low value of Q4is manually given to these frames.

Analyzer 46 is a frame matching analyzer that evaluates each pair ofcombined frames with a fitness value Q5 in terms of their degree ofmatch between the combined frames. The degree of match between frames ofeach pair is determined by an average value of correlation valuesobtained by the parallax calculator 29 for each frame pair as follows:Q5=AVs  (10)

As will be described later, the parallax calculator 29 defines windows(the size of a few pixels) in corresponding positions of the frames of apair and obtains correlation values between the windows. Frame matchinganalyzer 46 averages the correlation values (between −1 and 1) obtainedby the parallax calculator 29. If the average value of correlationsbetween frames of a pair is high, it is considered that there is a highdegree of match between the frames and the frame pair is evaluated witha high fitness value Q5.

It is seen from the foregoing that, for each frame pair, a set offitness values Q1˜Q5 is obtained. The same process is repeated until aplurality of sets of fitness values Q1˜Q5 are obtained from all framepairs formed by the frame combiner 41.

Specifically, the decision module 40 calculates the following Equationby respectively weighting the fitness values of a frame pair to obtain atotal value of weighted fitness value as a quality value Q of the framepair:Q=w ₁ Q1+w ₂ Q2+w ₃ Q3+w ₄ Q4+w ₅ Q5  (11)where, w₁ through w₅ are weight values for the analyzers 42 through 46.One example of the weight values is:w₁=10 and w₂=w₃=w₄=w₅=1

If the frame pair selector 28 is provided with only one analyzer, i.e.,the geometric condition analyzer 42, the weight value w₁ is set equal tounity and all other weight values are set equal to zero.

Decision module 40 repeats the same process on all frame pairs suppliedfrom all analyzers to produce their quality values Q and selects one ofthe frame pair having the highest quality value.

As shown in FIG. 6, the parallax calculator 29, connected to the outputof the frame pair selector 28, includes an interpolator 61, a framealigner 62 and a correlation calculator 63. If frames of differentresolutions are selected as a best pair by the frame pair selector 28,the frame of lower resolution is supplied to the interpolator 61 toimprove its resolution so that it is equal to the resolution of theother frame. The frames are then supplied to the frame aligner 62, wherethe frames are aligned so that they are parallel with an epipolar lineof a stereoscopic image. The aligned frames are fed into the correlationcalculator 63.

As shown in FIG. 7, the correlation calculator 63 segments each of thealigned frames into rectangular windows of several pixels on each sideand takes correlation between windows A and B of corresponding positionsto determine a correlation value r_(j,k) as follows:

$\begin{matrix}{r_{j,k} = \frac{{\sum\limits_{j}\;{\sum\limits_{k}^{\;}\;\left\{ {\left( {{I\left( {A,j,k} \right)} - {M(A)}} \right)\left( {{I\left( {B,j,k} \right)} - {M(B)}} \right)} \right\}}}\;}{\begin{matrix}\sqrt{\sum\limits_{j}\;{\sum\limits_{k}^{\;}\;\left\{ {{I\left( {A,j,k} \right)} - {M(A)}^{2}} \right\}}} \\\sqrt{\sum\limits_{j}\;{\sum\limits_{k}\;\left\{ {{I\left( {B,j,k} \right)} - {M(B)}^{2}} \right\}}}\end{matrix}}} & (12)\end{matrix}$where, I(A, j, k) is the pixel value of point (j, k) of the window A,I(B, j, k) is the pixel value of point (j, k) of the window B, and M(A)is the average of pixel values of window A, and M(B) is the average ofpixel values of window B.

By successively shifting the positions of the windows A and B pixel bypixel and calculating correlation values, the analyzer 46 seeks relativepositions of windows A and B where there is a peak or maximumcorrelation value. When such positions are detected, the analyzer 46produces output data indicating the corresponding relationship betweenthe center pixel of window A and the center pixel of window B. The aboveprocess is repeated for all windows of the frames to obtain a pluralityof frame-to-frame corresponding relationships (i.e., line-of-sightvectors) as parallax data.

On the other hand, the parallax data are obtained from all frame pairssupplied from the frame pair selector 28 are fed back to the framematching analyzer 46. When this occurs, the decision module of the framepair selector 28 receives all sets of weight data Q1 to Q5 from theanalyzers 42 to 46 and selects a best frame pair and commands thecorrelation calculator 63 to supply the parallax data of the best framepair to the parallax-terrain converter 30.

In more detail, as previously described with reference to FIG. 5, ifpixels p₁₁ and p₂₂ correspond to each other, the intersection point c₁of line-of-sight vectors v₁₁ and v₂₂ lies on a surface feature of thetarget area. In order to obtain a plurality of such intersection pointsin a three-dimensional coordinate system to produce topographic data,this conversion process is repeated for all line-of-sight vectorsrepresented by the parallax data to describe all surface features of thetarget area.

Parallax-terrain converter 30 performs a parallax-terrain conversionprocess by using the line-of-sight vectors indicated by the parallaxdata of the selected frame pair to produce terrain data which representssurface features of the target area.

Topographic data is supplied from the parallax-terrain converter 30 tooutput means 31 such as display or memory.

Due to the provision of the frame pair selector 28, picture frames takenby any pair of multiple image sensors can be selected for producingtopographic data, eliminating the inability to produce topographic datadue to the absence of stereoscopic images and enabling the selection ofbest stereoscopic images for particular purposes.

FIG. 8 is a flowchart which summarizes the operation of the firstembodiment of the present invention. At step 81, the satellites 11 and12 are encircling the earth to capture photographic images of variousregions of the earth on predetermined schedule. The captured images arescanned and transmitted to the earth station 14 and stored in storagemedium 27 (step 82). In response to demand for creating topographicdata, a plurality of frame pairs containing a target area are selectedfrom the storage medium (step 83). Parallax is calculated between theframes of each selected pair (step 84). When parallax is calculated forall frame pairs, a best frame pair is selected (step 85). Line-of-sightvectors indicated by the parallax data of the best frame pair are usedto produce topographic data (step 86) and utilized for display, storageor transmission (step 87).

In the first embodiment of this invention, topographic data is producedexclusively from the stored data in the storage medium 27. Ifappropriate frames are not available in the storage medium, notopographic data is obtained.

A second embodiment of the present invention, shown in FIG. 9, isintended to solve this problem. In FIG. 9, parts corresponding to thosein FIG. 2 are marked with the same numerals and the description thereofis omitted. In this embodiment, the system additionally includes animage sensing scheduler 90 which is connected to the frame pair selector28 and to the RF transceiver 25. The decision module of frame pairselector 28 checks to see if there is no usable frame or if the parallaxdata of the best frame pair is not usable. In either case, the framepair selector 28 instructs the image sensing scheduler 91 to proceedwith the formulation of an image sensing schedule. According to theformulated schedule, the scheduler 90 sends a sensing command signal toone or more earth observation satellites 91 via the RF transceiver 25.

As shown in FIG. 10, the scheduler 90 includes a decision module 100, asatellite selector 101, a geometric condition analyzer 102, a filteringcondition analyzer 103, a sunlight condition analyzer 104, atime-difference analyzer 105 and a frame matching analyzer 106.

Satellite selector 101 is responsive to the instruction from the framepair selector 28 to select one or more earth observation satelliteswhich cover the target area and send picture frames within a scheduledinterval of time, which may be ten minutes or as long as several months.If a satellite is flying over the same area several times during a knowntime interval, the image sensor of the same satellite is treated as aseparate sensor as long as the pictures are captured at different anglesto the target area. According to the instruction from the frame pairselector 28, the satellite selector 101 receives picture frames of theselected satellites from the RF transceiver 25 and supplies the receivedframes to the modules 102, 103, 104 and analyzers 105, 106.

Geometric condition analyzer 102 combines the picture frames suppliedfrom the satellite selector 101 into a plurality of pairs of frames thatcompose stereoscopic images and supplies the pairs of combined frames tothe other analyzers 103 to 106. Analyzer 102 further informs thedecision module 100 of the identifiers of the satellites from which thepaired picture frames are obtained. As described previously with respectto the geometric condition analyzer 42, the geometric condition analyzer102 calculates the quantum errors of the combined frames of eachstereoscopic pair and assigns a fitness value Q6 to each frame pair sothat a highest value Q6 is given to a frame pair of smallest quantumerror.

The operation of the geometric condition analyzer 101 for detectingquantum errors between two picture frames will be described withreference to FIG. 11.

Assume that satellites 11 and 12 are selected by the satellite selector101, respectively encircling the earth on orbits 16 and 17 that coverthe target area 13. A first set of sensing points p₁₋₁˜p₁₋₅ areestablished at intervals along the orbit 16, and a second set of sensingpoints p₂₋₁˜p₂₋₆ are established at intervals along the orbit 17.Picture frames at the sensing points are then paired between the firstand second sets, such as between point p₁₋₃ and point p₂₋₁, for example.For each pair of sensing points, a quantum error is then calculatedbetween the frames at the sensing points and the pair of sensing pointsis evaluated with a fitness value Q6 as follows:Q6=q _(—)6/E _(h)+1/E _(v)  (13)where q_6 is a positive weight value which is usually equal to unity.High Q6 value is assigned to a frame pair if the horizontal and verticalcomponents of the quantum error are small.

If more than two satellites are selected, sensing points are establishedin the same manner as discussed above. If only one new picture frame isdesired, using a stored frame as its companion, Equation (13) iscalculated by assuming that there is only one fixed sensing point forone of the satellites.

Filtering condition analyzer 103, the sunlight condition analyzer 104,the time difference analyzer 105 and the frame matching analyzer 106correspond respectively to the filtering condition analyzer 43, sunlightcondition analyzer 44, time difference analyzer 45 and frame matchinganalyzer 45, and operate in like manner to that described previously toproduce fitness values Q7, Q8, Q9 and Q10.

Decision module 100 makes a decision on the fitness values Q6˜Q10 ofeach frame pair and produces a total of weighted fitness values (qualityvalue) Q of the frame pair as follows:Q=w ₆ Q6+w ₇ Q7+w ₈ Q8+w ₉ Q9+w ₁₀ Q10  (14)where, w₆˜w₁₀ are weight values of the corresponding fitness values Q6to Q10. If use is made of only the geometric condition analyzer 102, theweight value w₆ is set equal to 1 and all the other weight values areset to zero.

Based on the total fitness value Q, the decision module 100 formulatesan image sensing schedule. The schedule includes data identifyingsatellites to be used, sensing positions, sensing times and filteringconditions. According to the schedule, the decision module 100 sends acommand signal to one or more satellites through the transmitter 92.

FIG. 12 is a flowchart which summarizes the operation of the secondembodiment of the present invention. At step 121, frame pair selectionis performed to select a plurality of pairs of frames from the storagemedium 27. At decision step 122, decision is made as to whether or notusable frames are available. If the selected frames are usable forproducing topographic data, flow proceeds to step 123 to performparallax calculation and select a best frame pair. At step 124, decisionis made as to whether or not parallax data of the best frame pair isusable. If the decision is affirmative, flow proceeds to step 125 toperform parallax-terrain conversion to produce topographic data which isoutput to a display or the like (step 126).

If the decision at step 122 or 124 is negative, flow proceeds to step127 to formulate an image sensing schedule and transmit a command signalto one or more satellites (step 128) and returns to step 121.

1. A topographic data processor comprising: frame selecting means forselecting a pair of frames from a plurality of candidate picture framesof a target area captured from high-altitude positions, said selectedpair of frames constituting a stereoscopic image of said target area;scheduling means for selecting at least one airborne image sensor if anappropriate frame is not available in said plurality of candidateframes, sensing picture frames from the selected image sensor, wherebysaid frame selecting means uses the sensed frames to select a pair offrames; means for determining a parallax between the frames selected bythe frame selecting means and producing therefrom a first plurality ofline-of-sight vectors and a second plurality of line-of-sight vectors;and means for converting said first and second pluralities ofline-of-sight vectors to topographic data, a plurality of receivedpicture frames being combined to form a plurality of pairs of sensedframes, each of said plurality of pairs of sensed frames constituting astereoscopic image of said target area, each of said plurality of pairsof sensed frames being evaluated with a fitness value indicative offitness of each of said pairs of sensed frames for topographicmeasurement of said target area, said selected pair of frames having ahighest fitness value of said plurality of pairs of sensed frames.
 2. Atopographic data processor as claimed in claim 1, wherein said frameselecting means comprises: means for combining the received pictureframes to form said plurality of pairs of sensed frames; evaluatingmeans for evaluating each pair of sensed frames with said fitness value;and scheduling means for producing a schedule for sensing picture framesfrom one or more airborne image sensor based on fitness values obtainedfrom all said pairs of sensed frames.
 3. A topographic data processor asclaimed in claim 2, wherein said evaluating means comprises a geometriccondition analyzer for analyzing said pairs of sensed frames in terms oftheir geometric condition and evaluating said pairs of frames with saidfitness value which is inversely proportional to quantum errors betweenthe frames of each said pair.
 4. A topographic data processor as claimedin claim 3, wherein said evaluating means further comprises filteringcondition analyzing means for analyzing each of said pairs of sensedframes in terms of filtering condition and evaluating each said pair ofsensed frames with said fitness value which is representative offiltering characteristics of image sensors.
 5. A topographic dataprocessor as claimed in claim 3, wherein said evaluating means furthercomprises sunlight condition analyzing means for analyzing each of saidpairs of sensed frames in terms of sunlight condition and evaluatingeach said pair of sensed frames with said fitness value which isrepresentative of degree of similarity in shadow and shading effectsbetween the frames of each said pair.
 6. A topographic data processor asclaimed in claim 3, wherein said evaluating means further comprises timedifference analyzing means for analyzing each of said pairs of sensedframes in terms of time difference and evaluating each said pair ofsensed frames with said fitness value which is inversely proportional toa time difference between the instant one of the frames of said eachpair is captured and the instant the other frame is captured.
 7. Atopographic measurement system comprising: at least one image sensormounted on a vehicle flying over a target area; means for sensing aplurality of picture frames at different positions by using said imagesensor; means for selecting a pair of frames from said plurality ofpicture frames, said pair of frames constituting a stereoscopic image ofsaid target area; means for determining a parallax between the selectedpair of frames and producing therefrom a first plurality ofline-of-sight vectors and a second plurality of line-of-sight vectors;and means for converting said first and second pluralities ofline-of-sight vectors to topographic data, said plurality of pictureframes being combined to form a plurality of pairs of frames, each ofsaid plurality of pairs of frames constituting a stereoscopic image ofsaid target area, each of said plurality of pairs of frames beingevaluated with a fitness value indicative of fitness of each of saidplurality of pairs of frames for topographic measurement of said targetarea, said selected pair of frames having a highest fitness value ofsaid plurality of pairs of frames.
 8. A topographic measurement systemas claimed in claim 7, wherein said frame selecting means comprises:frame combining means for combining said plurality of picture framesinto said plurality of pairs of frames; evaluating means for evaluatingeach of said plurality of pairs of frames with said fitness value; anddecision means for selecting one of said plurality of pairs of framesbased on fitness values obtained from said plurality of pairs of frames.9. A topographic measurement system as claimed in claim 8, wherein saidevaluating means comprises: a geometric condition analyzer for analyzingsaid plurality of pairs of frames in terms of their geometric conditionand evaluating each of said plurality of pairs of frames with saidfitness value proportional to their image resolution; and decisionmaking means for making a decision on the fitness values obtained fromall of said plurality of pairs of frames and selecting one of saidplurality of pairs of frames having said highest fitness value.
 10. Atopographic measurement system as claimed in claim 9, wherein saidparallax determining means determines a parallax between the frames ofsaid selected pair of frames, and wherein said evaluating means furthercomprises frame matching analyzing means for analyzing said plurality ofpairs of frames in terms of degree of match between each frame of eachof the paired frames and evaluating each of said plurality of pairs offrames with said fitness value proportional to an average value ofpoint-to-point correlations between each frame of said paired frames,wherein said decision making means produces a total value of the fitnessvalues of each of said plurality of pairs of frames and selects one ofsaid plurality of pairs of frames having the highest total value.
 11. Atopographic measurement system as claimed in claim 8, wherein saidparallax determining means comprises: frame aligning means for aligningthe frames of said selected pair of frames in orientation; andcorrelation calculating means for calculating point-to-pointcorrelations between the aligned frames.
 12. A topographic measurementsystem as claimed in claim 10, wherein said parallax determining meanscomprises: frame aligning means for aligning the frames of said selectedpair of frames in orientation; and correlation calculating means forcalculating point-to-point correlation values between the aligned framesand supplying the calculated correlation values to said frame matchinganalyzing means, and wherein the frame matching analyzing meanscalculates said average value of point-to-point correlations from thecorrelation values supplied from the correlation calculating means. 13.A topographic measurement system as claimed in claim 10, wherein saidparallax determining means further comprises an interpolator forinterpolating one of the selected pair of frames before said frames arealigned in orientation so that said frames of said selected pair haveequal value of resolution.
 14. A topographic measurement system asclaimed in claim 8, wherein said evaluating means further comprisesfiltering condition analyzing means for analyzing each of said pluralityof pairs of frames in terms of filtering condition and evaluating eachof said plurality of pairs of frames with said fitness value which isrepresentative of filtering characteristics of image sensors.
 15. Atopographic measurement system as claimed in claim 8, wherein saidevaluating means further comprises sunlight condition analyzing meansfor analyzing each of said plurality of pairs of frames in terms ofsunlight condition and evaluating each of said plurality of pairs offrames with said fitness value which is representative of degree ofsimilarity in shadow and shading effects between the frames of each ofsaid plurality of pairs of frames.
 16. A topographic measurement systemas claimed in claim 8, wherein said evaluating means further comprisestime difference analyzing means for analyzing each of said plurality ofpairs of frames in terms of time difference and evaluating each of saidplurality of pairs of frames with said fitness value which is inverselyproportional to a time difference between the instant one of the framesof each respective one of said plurality of pairs of frames is capturedand the instant the other frame of said each respective one of saidplurality of pairs of frames is captured.
 17. A topographic measurementsystem as claimed in claim 8, further comprising a storage medium forstoring a plurality of picture frames captured by airborne imagesensors, wherein said selecting means selects said selected pair offrames from said storage medium.
 18. A topographic measurement system asclaimed in claim 17, wherein said frame combining means includes areaselecting means for selecting frames covering said target area from allframes stored in said storage medium.
 19. A topographic measurementsystem as claimed in claim 7, further comprising an image sensingscheduler comprising: sensor selecting means for selecting at least oneairborne image sensor if an appropriate pair of frames is not availableto constitute said stereoscopic image of said target area from the imagesensor; frame combining means for combining the plurality of pictureframes to form said plurality of pairs of frames; evaluating means forevaluating each of said plurality of pairs of frames with said fitnessvalue; and scheduling means for producing a schedule for selecting oneor more airborne sensors based on said fitness values obtained from allof said plurality of pairs of frames.
 20. A topographic measurementsystem as claimed in claim 19, wherein said evaluating means of saidscheduler comprises a geometric condition analyzer for analyzing saidplurality of pairs of frames in terms of their geometric condition andevaluating each of said plurality of pairs of frames with said fitnessvalue which is inversely proportional to quantum errors between theframes of each respective one of said plurality of pairs of frames. 21.A topographic measurement system as claimed in claim 20, wherein saidevaluating means of the scheduler further comprises filtering conditionanalyzing means for analyzing each of said plurality of pairs of framesin terms of a filtering condition and evaluating each of said pluralityof pairs of frames with said fitness value which is representative offiltering characteristics of image sensors.
 22. A topographicmeasurement system as claimed in claim 20, wherein said evaluating meansof the scheduler further comprises sunlight condition analyzing meansfor analyzing each of said plurality of pairs of frames in terms ofsunlight condition and evaluating each of said plurality of pairs offrames with said fitness value which is representative of degree ofsimilarity in shadow and shading effects between the frames of eachrespective one of said plurality of pairs of frames.
 23. A topographicmeasurement system as claimed in claim 20, wherein said evaluating meansof the scheduler further comprises time difference analyzing means foranalyzing each of said plurality of pairs of frames in terms of timedifference and evaluating each of said plurality of pairs of frames withsaid fitness value which is inversely proportional to a time differencebetween the instant one of the frames of each respective one of saidplurality of pairs of frames is captured and the instant the other frameof said each respective one of said plurality of pairs of frames iscaptured.
 24. A topographic measurement system comprising: at least oneimage sensor mounted on a vehicle flying over a target area; means forsensing a plurality of picture frames of said target area at differentpositions by using said image sensor; frame selecting means forselecting a pair of frames from said plurality of picture frames of saidtarget area, said selected pair of frames constituting a stereoscopicimage of said target area; scheduling means for selecting at least oneimage sensor if an appropriate pair of frames is not available in saidplurality of picture frames, sensing picture frames from the selectedimage sensor, whereby said frame selecting means uses the sensed pictureframes to select a pair of frames; means for determining a parallaxbetween the pair of frames selected by the frame selecting means andproducing therefrom a first plurality of line-of-sight vectors and asecond plurality of line-of-sight vectors; and means for converting saidfirst and second pluralities of line-of-sight vectors to topographicdata, said plurality of picture frames being combined to form aplurality of pairs of frames, each of said plurality of pairs of framesconstituting a stereoscopic image of said target area, each of saidplurality of pairs of frames being evaluated with a fitness valueindicative of fitness of each of said plurality of pairs of frames fortopographic measurement of said target area, said selected pair offrames having a highest fitness value of said plurality of pairs offrames.
 25. A topographic measurement system as claimed in claim 24,wherein said scheduling means comprises: frame combining means forcombining said plurality of picture frames to form said plurality ofpairs of frames; evaluating means for evaluating each of said pluralityof pairs of frames with said fitness value; and decision means forproducing a schedule for sensing picture frames from one or moreairborne image sensors based on fitness values obtained from all of saidplurality of pairs of frames.
 26. A topographic measurement system asclaimed in claim 25, wherein said evaluating means comprises a geometriccondition analyzer for analyzing said plurality of pairs of frames interms of their geometric condition and evaluating said plurality ofpairs of frames with said fitness value which is inversely proportionalto quantum errors between the frames of each respective one of saidplurality of pairs of frames.
 27. A topographic measurement system asclaimed in claim 26, wherein said evaluating means further comprisesfiltering condition analyzing means for analyzing each of said pluralityof pairs of frames in terms of filtering condition and evaluating eachof said plurality of pairs of frames with a fitness value which isrepresentative of filtering characteristics of image sensors.
 28. Atopographic measurement system as claimed in claim 26, wherein saidevaluating means further comprises sunlight condition analyzing meansfor analyzing each of said plurality of pairs of frames in terms ofsunlight condition and evaluating each of said plurality of pairs offrames with a fitness value which is representative of degree ofsimilarity in shadow and shading effects between the frames of eachrespective one of said plurality of pairs of frames.
 29. A topographicmeasurement system as claimed in claim 26, wherein said evaluating meansfurther comprises time difference analyzing means for analyzing each ofsaid plurality of pairs of frames in terms of time difference andevaluating each of said plurality of pairs of frames with a fitnessvalue which is inversely proportional to a time difference between theinstant one of the frames of each respective one of said plurality ofpairs of frames is captured and the instant the other frame of said eachrespective one of said plurality of pairs of frames is captured.